(1) Field of the Invention
This invention relates to a method for treating metallic or ceramic surfaces kept at a high temperature of 200.degree. C. or above. More particularly, it relates to a method for treating uniformly metallic or ceramic surfaces comprising applying a lubricating composition comprising 50% by weight or more of a polyalkylene glycol or a derivative thereof thermally decomposable at 150.degree. C. or above in air with at least one specific fluorine-containing compound having a fluoroalkyl group in the molecule to the metallic or ceramic surfaces ordinarily used at 200.degree. C. or above, and treating the metallic or ceramic surfaces while thermally decomposing the polyalkylene glycol or derivative thereof.
(2) Description of the Prior Art
It is well known that polyalkylene glycols have hithereto been used as a lubricant used in high-temperature parts such as internal combustion engines, or dryers, cement roasting furnaces, glass smelting furnances, or the like since lubricants consisting essentially of polyalkylene glycols generally have improved lubricity at low temperatures or produce less residues as a sludge than any other lubricant on decomposition at a relatively high temperature. Polyalkylene glycols or derivatives thereof, however, are readily oxidized at high temperatures as is generally well known, and an antioxidant even in a large amount is incapable of providing the effect thereof for a long term. Therefore, large amounts of sludges are rather formed. In any case, polyalkylene glycols or derivatives thereof are usually unfit for use at high temperatures. For example, the following facts are known:
The decomposition starts by heating at 150.degree. C. or above in air for 24 hours. The higher the temperature or the higher the content of the copolymerized polyprolylene oxide in polyalkylene glycols, the higher is the decomposition rate. Thus, sludges are formed by the decomposition, and the lubricity is remarkably deteriorated. Therefore, not only in the case of polyalkylene glycols but in general, additives are known for solubilizing and dispersing sludge formed on metallic surfaces to reduce the disadvantages caused by the formation of the sludges even a little, for example, to prevent the sticking of sludges to the metallic surfaces.
Examples of the additives include copolymers such as lauryl methacrylate, diethylaminoethyl methacrylate and the like lately known as ashless high polymeric additives. Methods using the additives may be effective for applications wherein the metallic surfaces are always dipped in a solution at a relatively low temperature and locally or temporarily heated. It has been suggested that metallic surfaces be coated with an adsorbed film of sulfonates or phosphates to prevent the deposition of soot or resins thereon. However, the following disadvantages are usually caused:
All these methods are ineffective in applications wherein the metallic surfaces are always used at a high temperature of 200.degree. C. or above due to the thermal decomposition of the additives as such, and the formation of sludges is rather remarkably increased. The sticking of sludges on the metallic surfaces is also increased to deteriorate the lubricity, whereby cleaning should be frequently carried out.
On the other hand, silicone oil, mineral oil paraffin wax, fatty acid derivatives, talc, mica and the like as solid powders are often described as release agents in a relatively low temperature region from the viewpoint of improving only the release properties of the metallic surfaces. The term "release properties" herein used means the properties of removing sludges from surfaces. Mineral oil, however, will be not only fit for the use at 200.degree. C. or above but also emit much smoke and form large amounts of sludges after the lapse of a long time. Therefore, the mineral oil is quite unfit for practical use. On the other hand, silicone oil, modified silicone oil and fluorine-containing oil have the thermal stability and good release action even at a high temperature, however, will form rubber-like sludges at a temperature of 200.degree. C. or above or pile up successively to lose the effect thereof. The above-mentioned sludges will become tarry, and stain the surroundings remarkably or too great staining on the surfaces of products will often make the attainment of the object in improving the release properties of the metallic surfaces difficult. Futhermore, the often proposed salts of fatty acids will cause the danger of corrosion on the metallic surfaces due to the hydrolysis, dissociation and liberation of the fatty acids. For example, compounds such as esters of isethionic acid with fatty acids are regarded as good in the release properties at low temperature; however, it is difficult to keep the compounds stable and last the effect thereof. Corrosive action is increased due to the decomposition, and the formation and successive piling up of sludges cannot be easily avoided. Thus, it is possible to guess the importance of smooth continuous use without the continuous piling up of sludges on the metallic surfaces at 200.degree. C. or above. The treatment of the metallic surfaces by coating the surfaces with Teflon.RTM., and baking the resulting Teflon.RTM. coat to prevent the piling up or sticking of foreign substances is known in recent times; however, the treated surfaces are ordinarily used mainly at about 100.degree. C. The surfaces treated with Teflon.RTM. cannot be easily maintained as they are at 200.degree. C. or above at all.
The Teflon.RTM. coat has basic disadvantages as follows:
The abrasion resistance is very low, and scratches are readily formed on the surface. The coat is unfit for practical use at a temperature of 200.degree. C. or above by any means.